In optical wavelength division multiplex (WDM) passive optical networks, which are commonly applied to realized fiber-to-the home access structures, a plurality of optical network units (ONU) is connected to a central node, also referred to as optical line terminal (OLT), via a remote node (RN). Each ONU connects at least one end-user to the RN. Generally, a single fiber is used for connecting the RN to the ONUs in order to save fiber usage. Using a single-fiber connection between the OLT and the RN further reduces the fiber usage. However, PON configurations with dual-fiber connection between the OLT and the RN are also widely used. Often, a protection mechanism, especially a fiber-protection method and hardware, may be implemented for the transmission link between the OLT and the RN.
PONs enable a bi-directional point-to-point connection between each ONU and the OLT using dedicated optical channels, i.e. for each point-to-point connection a pair of downstream and upstream signals having predetermined optical wavelengths is used. In general, the downstream channel signal transmitted from the OLT to the respective ONU and the upstream channel signal transmitted in the reverse direction may have identical or differing optical wavelengths. The plurality of downstream and upstream optical channel signals is transmitted as a combined WDM signal within the transmission link between the OLT and the RN.
One of the main challenges when deploying this PON technology in access networks arises from the wavelength-assignment problem of WDM transmitters and the costs related therewith. The costs of installation, administration, and maintenance for a PON can be drastically reduced if at least the ONUs reveal a so-called “colorless”, i.e. non-wavelength-specific, design. Typical low-cost solutions for colorless ONUs are based on the use of reflective optical transmitters like semiconductor optical amplifiers (RSOA), injection-locked Fabry-Perot laser diodes (IL-FP-LD) or reflective electro-absorption modulators with integrated semiconductor optical amplifiers (REAM-SOA) as optical transmitter components.
In a PON design using such reflective optical transmitters, the downstream (channel) signals are reused in the ONUs in order to create an upstream signal having the same wavelength. Thus, such a system for bi-directionally transmitting digital optical signals over an optical transmission link is referred to as wavelength-reuse transmission systems.
In a WDM-PON with wavelength reuse, the (optical) downstream (channel) signals must not use standard non-return-to-zero (NRZ) on-off keying (OOK) because for levels “0” sent, upstream modulation is impossible or subject to severe penalties. Here and in the following description, a level “0” or a logical value “0” of an amplitude-modulated optical signal shall be understood in such a way that an optical power of essentially zero is transported within the respective symbol interval. Therefore, a modulation format other than NRZ-OOK must be chosen for the downstream signals. However, other modulation formats always involve additional effort and costs.
A known approach for a WDM-PON applying wavelength-reuse is based on using frequency-shift keying (FSK) for the downstream signal, wherein this signal reveals an essentially constant envelope, and a standard OOK modulation format for the upstream signal. Instead of FSK any other modulation format may be used that produces a downstream signal having an essentially constant envelope, like any phase shift keying (FSK) format.
Further, it has been proposed to use an inverse-return-to-zero (IRZ) OOK modulation format for the downstream signal and an RZ OOK modulation format for the related upstream signal.
However, all these solutions require costly components, e.g. FSK demodulators or IRZ/RZ pulse shaping within the ONU, or increase the bandwidth necessary to transmit the downstream or even the downstream and the upstream signals and thus increase the costs for the components necessary to process the respective signals.